1986 Chalfant Valley earthquake

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1986 Chalfant Valley earthquake
Relief map of California.png
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Fallon
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Sacramento
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Oceanside
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Las Vegas
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Santa Cruz
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Palm Springs
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Death Valley
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Coalinga
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UTC  time1986-07-21 14:42:28
ISC  event 491308
USGS-ANSS ComCat
Local dateJuly 21, 1986 (1986-07-21)
Local time07:42:28 PDT
Magnitude6.2 Mw [1]
Depth10 km (6.2 mi) [1]
Epicenter 37°32′N118°25′W / 37.54°N 118.42°W / 37.54; -118.42 Coordinates: 37°32′N118°25′W / 37.54°N 118.42°W / 37.54; -118.42 [1]
Type Strike-slip
Areas affected Eastern California
United States
Total damage$2.7 million USD [2]
Max. intensity VI (Strong) [2]
Peak acceleration0.46 g [3]
Foreshocks5.7 ML July 20 at 14:29 [3]
Casualties2 injured [2]

The 1986 Chalfant Valley earthquake struck southern Mono County near Bishop and Chalfant, California at 07:42:28 Pacific Daylight Time on July 21. With a moment magnitude of 6.2 and a maximum Mercalli intensity of VI (Strong), the shock injured two people and caused property damage estimated at $2.7 million in the affected areas. There was a significant foreshock and aftershock sequence that included a few moderate events, and was the last in a series of three earthquakes that affected southern California and the northern Owens Valley in July 1986.

Contents

Strong motion stations at the Long Valley Dam and an instrumented building in Bishop captured light to moderate readings from the event. The faulting reached the surface of the ground, but the maximum amount of measurable slip was limited to about 11 centimeters (4.3 in) of strike-slip motion along the White Mountain Fault Zone. Other destructive and surface-rupturing earthquakes, as well as large volume volcanic eruptions, have occurred in the area. Although the series of shocks occurred within the White Mountain seismic gap, the region continues to have potential for a very large earthquake.

Preface

July 1986 was an unusually active month for moderate to strong earthquakes in California, with three events occurring in less than two weeks, each with mild to moderate effects. The first of these shocks came on July 8 with a M6.0 event on the Banning fault near Palm Springs and the second event occurred off the coast of Oceanside as a M5.8 shock on July 13. While the earthquake off the southern California coast occurred in an area thought to be capable of generating a tsunami, the earthquake near Palm Springs occurred on a portion of the southern San Andreas Fault system that has been designated a seismic gap, and is a likely location for a very large earthquake. The same seismic gap theory (regarding the potential for a future large earthquake) had also been presented for the White Mountains area near Chalfant. [4]

Tectonic setting

The Owens Valley region Wpdms shdrlfi020l owens valley.jpg
The Owens Valley region

The Owens Valley, located at the western boundary of the Basin and Range Province, is confined by the Sierra Nevada mountain range to the west and the White-Inyo Mountains to the east. The valley was brought about by active tectonics over the last 2–4 million years, and was the scene of a very large earthquake in 1872 that generated surface rupture from Lone Pine in the south to as far north as Big Pine, a distance of approximately 100 kilometers (62 mi). Other large, surface rupturing events in 1915, 1932, and 1954 delineate the Eastern California–Central Nevada seismic zone. [5]

As no large earthquake has occurred between the southern extent of the 1932 Cedar Mountains (Nevada) earthquake's rupture and the northern extent of the 1872 Lone Pine earthquake's rupture in the south (a distance of 130 km (81 mi)), the zone has been labeled the White Mountain seismic gap. The 1934 M6.3 Excelsior Mountains earthquake and the 1986 Chalfant Valley event were several smaller earthquakes that have occurred within the gap, and both generated limited surface faulting and some surface cracking in the Volcanic Tableland, which was created .7 million years ago from a major volcanic eruption that also formed the Long Valley Caldera northwest of Bishop. An estimated 500 cubic kilometers of material (tephra) produced in the event covers the northern Owens Valley as a rocky landform. The surface of the layered plateau is known as Bishop Tuff and features fumarole mounds and hundreds of north-south oriented fault scarps, many of which are visible on topographic maps, via aerial photography, and satellite imagery. [5]

Foreshocks

The University of Nevada, Reno (UNR) and the United States Geological Survey (USGS) both operated seismograph networks in the region where the Chalfant Valley earthquake sequence occurred. The data that was captured was transmitted in real time to Reno, Nevada and Menlo Park, California and was digitized for later examination. The foreshock activity was found to have begun eighteen days prior to the main event on July 3, with the onset of a 3.5 event (local magnitude). Activity increased on July 18 with several doublets then reduced again in the hours prior to the primary M5.7 (intensity V (Moderate)) foreshock on July 20. All forty events that were recorded by the UNR and USGS stations before that event were within 2 km (1.2 mi) of its epicenter, and another 132 events that were considered aftershocks were documented in the remaining 24 hours leading up to the mainshock. One of these events, just over two hours after the primary foreshock, measured M4.7 with an intensity of IV (Light). [2] [6]

Earthquake

USGS ShakeMap for the event USGS Shakemap - 1986 Chalfant Valley earthquake.jpg
USGS ShakeMap for the event

The mainshock occurred at 07:42:26 on July 21 and caused the most damage in the sequence of events. The shock measured 6.2 Mw  as measured by the International Seismological Centre and originated at a depth of approximately 10.8 kilometers (6.7 mi). The focal mechanisms of the three largest events (the July 20 foreshock, the mainshock, and the largest aftershock) were primarily strike-slip with a minor amount of normal-slip and were especially well-constrained due to the dense array of seismometers in the area that were operated by the UNR and the USGS. The primary foreshock's preferred fault plane solution indicated left-lateral slip on a fault striking N25°E, but neither the mainshock nor the largest aftershock conformed with their right-lateral slip striking N25°W and N15°W respectively. [7]

Damage

In Bishop, windows were shattered and ceiling tiles fell in several buildings, and a portion of the brick façade at the First Sierra bank building fell onto the sidewalk. A Burger King restaurant had part of its ceiling come down. Northwest of Bishop, a portion of U.S. Route 395 was blocked temporarily by a landslide. Campers were briefly trapped at the Pleasant Valley Reservoir when a 150 ft (46 m) section of an access road was destroyed as a result of land movement. Both injuries that were reported (minor cuts and injuries from falling objects) occurred to the north in Chalfant Valley where 53 mobile homes were knocked off their foundations and two homes were destroyed. Nearly all the buildings in that small town were affected, with water and sewer lines broken there as well. [8]

Many of the mobile homes were able to be remounted on their foundations, but 18–20 of the homes were unable to be repaired. The overall damage from the event was compared with the other events in southern California. While the Oceanside shock caused $200,000 in damage, the Palm Springs event caused an estimated $8 million in damage. Damage in from the Chalfant Valley event was estimated at $2.7 million. [2] [9]

Surface faulting

Ground fractures that were deemed to be tectonic in nature were documented on about 10.5 kilometers (6.5 mi) of the White Mountain Fault Zone from Silver Creek to Piute Creek. The cracks trended 350° and a maximum right-lateral slip of about 11 centimeters (4.3 in) was measured, though other vertical (downslope) or extensional movement was also seen, especially on the 150 m (500 ft)-thick surface of the Bishop Tuff in the Volcanic Tableland. There, the largest displacements were seen in the vicinity of Casa Diablo Mountain. The cracks were found primarily in loose sand, which made measurements difficult. [10]

Intensity

Main shock Mercalli intensities
MMI Locations
VI (Strong) Bishop, Chalfant
V (Moderate) Coalinga, Death Valley
IV (Light) Fallon, Sacramento
III (Weak) Santa Cruz, Las Vegas, Oceanside
Stover & Brewer 1994 , pp. 47–50

In Bishop, intensity VI effects included broken windows and cracked walls in commercial and government buildings. Items fell from store shelves at grocery markets, streets were cracked, and wells had changes in the flow of water. The damage to the mobile homes in Chalfant was also consistent with intensity VI shaking, with some of the homes moving laterally as much as 18 inches. Also in this classification was damage to pipe supports along a portion of the Los Angeles Aqueduct between the Upper, Middle, and Control Gorge Power Plants southeast of Crowley Lake. Intensity V (Moderate) effects (including cracked windows and broken underground pipes) were present in the Central Valley, Death Valley, and Carson City, Nevada. [11] [12]

Strong motion

The foreshock, mainshock, and the two largest aftershocks were recorded by strong motion stations that were operated by the California Strong Motion Instrumentation Program (CSMIP) and the USGS. A total of 36 seismograms were captured from 11 stations, including a two-story steel frame building, the Long Valley Dam, and several free field stations. The Chalfant strong motion station recorded the largest horizontal accelerations for the foreshock, mainshock, and the July 21 M5.6 aftershock of 0.28 g, 0.46 g, and 0.17 g. The instruments at the building on North Main street in Bishop recorded all four shocks and indicated 0.25 g at ground level and 0.4 g on the roof, both during the mainshock. The station at the earthen Long Valley Dam had produced many quality recordings since being put into operation in 1979 and saw mainshock accelerations of 0.09 g on bedrock, and 0.24 g on an upper abutment. [13]

Aftershocks

The day of the main event two aftershocks occurred (5.6 at 14:51 and 5.4 at 22:07) but the initial shock was V (Moderate) on the Mercalli intensity scale and the second event was felt only. Ten days following the mainshock on July 31, a stronger aftershock occurred with a local magnitude of 5.8 and a maximum Mercalli intensity of VI (Strong). That aftershock was felt in California and western Nevada, broke windows and knocked items off shelves in Bishop, and light fixtures were damaged at the National Weather Service office. [2] [7]

See also

Related Research Articles

In seismology, an aftershock is a smaller earthquake that follows a larger earthquake, in the same area of the main shock, caused as the displaced crust adjusts to the effects of the main shock. Large earthquakes can have hundreds to thousands of instrumentally detectable aftershocks, which steadily decrease in magnitude and frequency according to a consistent pattern. In some earthquakes the main rupture happens in two or more steps, resulting in multiple main shocks. These are known as doublet earthquakes, and in general can be distinguished from aftershocks in having similar magnitudes and nearly identical seismic waveforms.

A foreshock is an earthquake that occurs before a larger seismic event and is related to it in both time and space. The designation of an earthquake as foreshock, mainshock or aftershock is only possible after the full sequence of events has happened.

1987 Whittier Narrows earthquake 1987 earthquake in southern California, United States

The 1987 Whittier Narrows earthquake occurred in the southern San Gabriel Valley and surrounding communities of Southern California, United States, at 7:42 a.m. PDT on October 1. The moderate magnitude 5.9 blind thrust earthquake was centered several miles north of Whittier in the town of Rosemead, had a relatively shallow depth, and was felt throughout southern California and southern Nevada. Many homes and businesses were affected, along with roadway disruptions, mainly in Los Angeles and Orange counties. Damage estimates ranged from $213–358 million, with 200 injuries, three directly-related deaths, and five additional fatalities that were associated with the event.

The 1952 Kern County earthquake occurred on July 21 in the southern San Joaquin Valley and measured 7.3 on the moment magnitude scale. The main shock occurred at 4:52 am Pacific Daylight Time, killed 12 people, injured hundreds more and caused an estimated $60 million in property damage. A small sector of damage near Bealville corresponded to a maximum Mercalli intensity of XI (Extreme), though this intensity rating was not representative of the majority of damage. The earthquake occurred on the White Wolf Fault near the community of Wheeler Ridge and was the strongest to occur in California since the 1906 San Francisco earthquake.

San Jacinto Fault Zone

The San Jacinto Fault Zone (SJFZ) is a major strike-slip fault zone that runs through San Bernardino, Riverside, San Diego, and Imperial Counties in Southern California. The SJFZ is a component of the larger San Andreas transform system and is considered to be the most seismically active fault zone in the area. Together they relieve the majority of the stress between the Pacific and North American tectonic plates.

The 1930 Salmas earthquake occurred on May 7 at 01:34:26 IRST in West Azerbaijan Province, Iran. The earthquake, which was among Iran's largest, measured 7.1 on the moment magnitude scale and had a maximum Mercalli intensity of IX (Violent). A damaging foreshock occurred fifteen hours prior to the main event and served as a warning to the people that felt it strongly. Reports from seismologists and seismological organizations indicate that up to 3,000 fatalities may have occurred in northwest Iran and southeast Turkey.

The 1992 Cape Mendocino earthquakes occurred along the Lost Coast of Northern California on April 25 and 26. The three largest events were the M7.2 thrust mainshock that struck near the unincorporated community of Petrolia midday on April 25 and two primary strike-slip aftershocks measuring 6.5 and 6.6 that followed early the next morning. The sequence encompassed both interplate and intraplate activity that was associated with the Mendocino Triple Junction, a complex system of three major faults that converge near Cape Mendocino. The total number of aftershocks that followed the events exceeded 2,000.

The 1948 Desert Hot Springs earthquake occurred on December 4 at 3:43 p.m. Pacific Standard Time with a moment magnitude of 6.4 and a maximum Mercalli intensity of VII. The shock was felt from the central coast of California in the north, and to Baja California in the south, and came at a time when earthquake research in southern California resumed following the Second World War. It was one of two events in the 20th century that have occurred near a complex region of the southern San Andreas Fault system where it traverses the San Gorgonio Pass and the northern Coachella Valley. Damage was not severe, but some serious injuries occurred, and aftershocks continued until 1957.

The 1979 Imperial Valley earthquake occurred at 16:16 Pacific Daylight Time on 15 October just south of the Mexico–United States border. It affected Imperial Valley in Southern California and Mexicali Valley in northern Baja California. The earthquake had a relatively shallow hypocenter and caused property damage in the United States estimated at US$30 million. The irrigation systems in the Imperial Valley were badly affected, but no deaths occurred. It was the largest earthquake to occur in the contiguous United States since the 1971 San Fernando earthquake eight years earlier.

The 1979 Coyote Lake earthquake occurred at 10:05:24 local time on August 6 with a moment magnitude of 5.7 and a maximum Mercalli Intensity of VII. The shock occurred on the Calaveras Fault near Coyote Lake in Santa Clara County, California and resulted in a number of injuries, including some that required hospitalization. Most of the $500,000 in damage that was caused was non-structural, but several businesses were closed for repairs. Data from numerous strong motion instruments was used to determine the type, depth, and extent of slip. A non-destructive aftershock sequence that lasted throughout the remainder of the month was of interest to seismologists, especially with regard to fault creep, and following the event local governments evaluated their response to the incident.

The 1957 San Francisco earthquake occurred on March 22 at 11:44:22 local time with a moment magnitude of 5.7 and a maximum Mercalli Intensity of VII. It was located just off the San Francisco Peninsula near the San Andreas Fault and was felt in a limited portion of Northern and Central California. There was a non-destructive foreshock and aftershock sequence that lasted for several months. With financial losses of around US$1 million, damage was considered minimal, with one death and forty injuries.

The 1986 North Palm Springs earthquake occurred on July 8 at 02:20:44 local time with a moment magnitude of 6.0 and a maximum Mercalli Intensity of VII. The shock occurred in a complex setting along the San Andreas Fault Zone where it bisects San Gorgonio Mountain and San Jacinto Peak at the San Gorgonio Pass and was the first in a series of three earthquakes that affected southern California and the northern Owens Valley in July 1986. Numerous strong motion instruments recorded the event, one of which showed relatively high accelerations. Between 29 and 40 people were injured, and financial losses were estimated to be in the range of $4.5 to 6 million.

The 1898 Mare Island earthquake occurred in Northern California on March 30 at 23:43 local time with a moment magnitude of 5.8–6.4 and a maximum Mercalli intensity of VIII–IX (Severe–Violent). Its area of perceptibility included much of northern and central California and western Nevada. Damage amounted to $350,000 and was most pronounced on Mare Island, a peninsula in northern San Francisco Bay. While relatively strong effects there were attributed to vulnerable buildings, moderate effects elsewhere in the San Francisco Bay Area consisted of damaged or partially collapsed structures, and there were media reports of a small tsunami and mostly mild aftershocks that followed.

The 2019 Ridgecrest earthquakes of July 4 and 5 occurred north and northeast of the town of Ridgecrest, California located in Kern County and west of Searles Valley. They included three initial main shocks of Mw magnitudes 6.4, 5.4, and 7.1, and many perceptible aftershocks, mainly within the area of the Naval Air Weapons Station China Lake. Eleven months later, a Mw  5.5 aftershock took place to the east of Ridgecrest. The first main shock occurred on Thursday, July 4 at 10:33 a.m. PDT, approximately 18 km (11.2 mi) ENE of Ridgecrest, and 13 km (8.1 mi) WSW of Trona, on a previously unnoticed NE-SW trending fault where it intersects the NW-SE trending Little Lake Fault Zone. This quake was preceded by several smaller earthquakes, and was followed by more than 1,400 detected aftershocks. The M 5.4 and M 7.1 quakes struck on Friday, July 5 at 4:08 a.m. and 8:19 p.m. PDT approximately 10 km (6 miles) to the northwest. The latter, now considered the mainshock, was the most powerful earthquake to occur in the state in 20 years. Subsequent aftershocks extended approximately 50 km (~30 miles) along the Little Lake Fault Zone.

In 1954, the state of Nevada was struck by a series of earthquakes that began with three M 6.0+ events in July and August that preceded the M 7.3 mainshock and M 6.9 aftershock, both on December 12. All five earthquakes remain some of the largest in the state, and the largest since the Cedar Mountain earthquake of 1932 and Pleasant Valley event in 1915. The earthquake was felt throughout much of the western United States.

The 1968 Borrego Mountain earthquake occurred on April 8, at 6:28 p.m. PST, near the unincorporated community of Ocotillo Wells in San Diego County. The moment magnitude 6.6 earthquake was assigned a maximum Modified Mercalli intensity (MMI) of VII. Shaking was felt in Nevada, and Arizona. It was the largest earthquake to strike California since 1952. Although the earthquake did cause major damage in Southern California, it was the subject of scientific interest because it displayed a phenomenon known as afterslip.

The 1995 Menglian earthquake or 1995 Myanmar–China earthquake occurred on 12 July at 05:46:43 local time in the Myanmar–China border region. The earthquake had an epicenter on the Myanmar side of the border, located in the mountainous region of Shan State. It registered 7.3 on the Chinese surface wave magnitude scale (Ms ) and 6.8 on the moment magnitude scale (Mw ). With a maximum Mercalli intensity assigned at VIII, the quake killed eleven people and left another 136 injured. Over 100,000 homes in both countries were destroyed and 42,000 seriously damaged. Some damage to structures were also reported in Chiang Mai and Chiang Rai, Thailand. The low death toll from this earthquake was attributed to an early warning issued prior to it happening. Precursor events including foreshocks and some seismic anomalies led to an evacuation of the area before the mainshock struck. It is thought to be one of the few successfully predicted earthquakes in history.

The 1941 Sa'dah earthquake or the Jabal Razih earthquake occurred on January 11 in the Razih District of Yemen. The earthquake had a surface wave magnitude of 5.8–6.5 and a shallow focal depth. Despite the moderate size of this earthquake, an estimated 1,200 people perished and at least 200 injured. With a maximum MSK-64 intensity assigned at VIII, it destroyed many villages and collapsed homes in the region of North Yemen.

The second shock in the 1962 Irpinia earthquake sequence was the largest and most destructive in a series of earthquakes in the southern Apennines. It occurred on August 21 at 18:19 CET, measuring Mw 6.15 and assigned a maximum intensity of IX (Violent). It was preceded by an Mw  5.68 foreshock, and followed by a 5.34 aftershock. The earthquakes resulted in nearly 20 fatalities and significant property losses.

The 1895 Charleston earthquake, also known as the Halloween earthquake, occurred on October 31, at 05:07 CST near Charleston, Missouri. It had an estimated moment magnitude of 5.8–6.6 and evaluated Modified Mercalli intensity of VIII (Severe). The earthquake caused substantial property damage in the states of Missouri, Illinois, Ohio, Alabama, Iowa, Kentucky, Indiana, and Tennessee. Shaking was widespread, being felt across 23 states and even in Canada. At least two people died and seven were injured.

References

  1. 1 2 3 ISC (2015), ISC-GEM Global Instrumental Earthquake Catalogue (1900–2009), Version 2.0, International Seismological Centre
  2. 1 2 3 4 5 6 Stover, C.W.; Coffman, J.L. (1993), Seismicity of the United States, 1568–1989 (Revised), U.S. Geological Survey Professional Paper 1527, United States Government Printing Office, pp. 97, 177
  3. 1 2 Smith, K.D.; Priestley, K.F. (2000), "Faulting in the 1986 Chalfant, California, Sequence: Local tectonics and earthquake source parameters" (PDF), Bulletin of the Seismological Society of America, 90 (4): 816, 824, Bibcode:2000BuSSA..90..813S, doi:10.1785/0119990129
  4. ↑ Pacheco, J.; Nabelek, J. (1988), "Source mechanisms of three moderate California earthquakes of July 1986", Bulletin of the Seismological Society of America, 78 (6): 1907–1909
  5. 1 2 Pinter, N. (1995), "Faulting on the Volcanic Tableland, Owens Valley, California", The Journal of Geology, 103 (1): 73–76, Bibcode:1995JG....103...73P, doi:10.1086/629723, JSTOR   30071137
  6. ↑ Smith, K.D.; Priestley, K.F. (1988), "The foreshock sequence of the 1986 Chalfant, California, earthquake", Bulletin of the Seismological Society of America, 78 (1): 173, 177, CiteSeerX   10.1.1.919.4371
  7. 1 2 Cockerham, R.S.; Corbett, E.J. (1987), "The July 1986 Chalfant Valley, California, earthquake sequence: Preliminary results", Bulletin of the Seismological Society of America, 77 (1): 280–285
  8. ↑ Ramos, George; Reich, Kenneth (July 22, 1986). "6.1 Quake Jars Eastern Sierra : State's 4th Temblor in Two Weeks Damages Homes, Traps Campers". Los Angeles Times .
  9. ↑ Ramos, George; Reich, Kenneth (July 23, 1986). "More Big Temblors Could Rock Sierra in Next Few Days". Los Angeles Times .
  10. ↑ Lienkaemper, J.J.; Pezzopane, S.K.; Clark, M.M.; Rymer, M.J. (1987), "Fault fractures formed in association with the 1986 Chalfant Valley, California, earthquake sequence: Preliminary report" (PDF), Bulletin of the Seismological Society of America, 77 (1): 297, 298, 301, 302
  11. ↑ Stover, C. W.; Brewer, L. R. (1994), United States earthquakes, 1986 (PDF), U.S. Geological Survey Bulletin 2089, United States Government Printing Office, pp. 47–50
  12. ↑ Brewer, L. (1989), The Intensity of the July 21, 1986, Chalfant Valley, California, Earthquake, Open-File Report 89-135, United States Geological Survey, pp. 4–7, 10
  13. ↑ Sherburne, R.W.; Parke, D.L.; Huang, M.J.; Shakal, A.F. (1988), CSMIP strong-motion records from the Chalfant Valley, California earthquakes, Report OSMS 86-06, California Strong Motion Instrumentation Program, California Division of Mines and Geology, pp. 1–3
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